This invention relates to magneto-optic recording systems and, in particular, to magneto-optic recording systems that use thin film recording layers to record, reproduce and erase information data.
In 1957, a study showed that a thermo-pen can be used to record information data on a MnBi thin film. A magnetic domain can be written by the thermo-pen on the MnBi film as a result of the effect of heat from the thermo-pen.
Subsequently, development of lasers provided impetus for a comprehensive study of the effect of lasers on MnBi thin films. However, the research was of little practical utility because studies of laser sources and applications had not reached an advanced stage at that time.
By the 1970's, optical information processing techniques were significantly more advanced and studies of thin film recording layers resumed. The thin film recording layers studied were characterized by use of an amorphous rare earth transition metal alloy. Such films included binary alloy thin films like gadolinium-iron (GdFe), terbium-iron (TbFe), dysprosium-iron (DyFe) and gadolinium-iron (GdFe). Thin film recording layers containing amorphous rare earth transition metals are disclosed in Japanese Patent Publication No. 56-37607.
It was later found that use of ternary and quaternary alloys improved the properties of thin film recording layers. Such alloys included gadolinium-terbium-iron (GdTbFe) and gadolinium-terbium-iron-cobalt (GdTbFeCo). These alloys and their use as thin film recording layers are disclosed in Japanese Laid Open Publications Nos. 56-126907 and 57-94948.
Magnetic thin film recording layers operate to reproduce binary information bits because of a phenomenon known as the magneto-optic effect. To record information data, the temperature dependence of the magnetic coercivity (H.sub.c) of the magnetic thin film recording layer is exploited. Magnetic coercivity is a measure of the force which must be applied to a magnetic field in order to reverse the magnetization of the materials.
To record an information bit on the thin film recording layer of a magneto-optic recording system, a small magnetic field bias is applied to the film and light flux from a laser is then focused on a small area (about one micrometer) of the film to heat that area to a temperature at which the magnetic coercivity of the film is less than the field bias or at which the area becomes paramagnetic.
The information bit is read by reflecting polarized light on the film surface. Rotation of the polarization of the reflected light is measured by transmission through a polarizor. This rotation is referred to as the magneto-optical Kerr effect. The rotation angle, also known as Kerr rotation angle, is measured in degrees or in minutes.
Erasure of the information bits is accomplished either by reversing the field bias and rewriting onto the film or by increasing the overall field bias until it exceeds the magnetic coercivity of the film.
Conventional magnetic thin film recording systems have several disadvantages. Specifically, the supply of heavy rare earth elements is limited. Furthermore, use of a large amount of only one specific heavy rare earth element is economically disadvantageous. For this reason, the cost of producing conventional types of magneto-optic recording systems using magnetic thin films is high.
Additionally, the magnetic thin films used in magneto-optic recording systems can be prepared by sputtering. An alloy target having a predetermined composition is primarily used as the sputtering target. However, heavy rare earth element and transition metal element alloys are very brittle. For this reason, it is difficult to produce a large alloy target.
In order to solve this problem, methods for disposing a rare earth element pellet on a transition metal sputtering target, for sputtering two separate targets of transition metal and rare earth metal simultaneously and alloying them on a substrate and for forming a composite target by pasting a transition metal and a rare earth metal together have been proposed. This last method is disclosed in Japanese Patent Laid Open Publication No. 51-63492. All of these methods are disadvantageous in that the composition of the ferromagnetic alloy thin film produced is not uniform and productivity is low.
Additionally, heavy rare earth transition metal thin films such as those used in conventional magneto-optic recording systems have poor weatherability. This means that various properties of the film deteriorate rapidly after the film is formed.
In heavy rare earth transition metal alloys, the temperature co-efficient of magnetization is large. This large temperature co-efficient causes a proportionately large change in Kerr rotation angle as a result of temperature changes. Consequently, reading properties of the thin film are unstable because of the temperature dependency of the Kerr rotation angle.
In GdFe and GdCo thin films, the Kerr rotation angle is larger and optical reproduction properties are better than for TbFe or GdTbFeCo. However, since the magnetic coercivity of GdFe and GdCo is small (several hundredths of an oersted), magnetic domains which are information bits are not stable. On the other hand, TbFe and GdTbFeCo thin films have large magnetic coercivity, and the magnetic domains of these films are stable. However, Kerr rotation angle is small and optical reproduction properties are not good.
As can be seen, in conventional methods for improving ferromagnetic thin film recording, reflectance of the thin film decreases as Kerr rotation angle increases. Since both high reflectance and large Kerr rotation angle are optimum, no fundamental improvement is achieved.
Accordingly it is desired to provide ferromagnetic thin film recording systems that overcome the disadvantages inherent in prior art systems.